1. Field of the Invention
The present invention relates to a laser irradiation apparatus used for crystallizing a semiconductor film. Moreover, the present invention relates to a method for manufacturing a semiconductor device.
2. Related Art
A thin film transistor (TFT) formed using a poly-crystalline semiconductor film is higher in mobility by double digits or more than a TFT formed using an amorphous semiconductor film, and has an advantage that a pixel portion and a peripheral driver circuit in a semiconductor display device can be integrally formed over the same substrate. The poly-crystalline semiconductor film can be formed over an inexpensive glass substrate by employing a laser annealing method.
As laser oscillators used in the laser annealing method, there are a pulsed laser oscillator and a continuous wave laser oscillator according to the oscillation method. The pulsed laser oscillator typified by an excimer laser has output power per unit time which is approximately 3 to 6 digits higher than that of the continuous wave laser oscillator. Therefore, the throughput of the laser irradiation can be increased by shaping a beam spot (an irradiation region irradiated with the laser beam in fact on the surface of the irradiation object) into a rectangular spot having a length of several cm on a side or a linear spot having a length of 100, mm or more with the use of an optical system. For this reason, the pulsed laser oscillator has been mainly employed for crystallizing the semiconductor film.
It is noted that a term of “linear” herein used does not mean a line in a strict sense but means a rectangle (or long ellipse) having a large aspect ratio. For example, a rectangular beam having an aspect ratio of 2 or more (preferably 10 to 10000) is referred to as a linear beam. It is to be noted that the linear is included in the rectangular.
The semiconductor film crystallized thus by the pulsed laser beam includes plenty of crystal grains whose positions and sizes are random. Unlike the inside of the crystal grain, an interface between the crystal grains (a crystal grain boundary) includes an infinite number of trapping centers and recombination centers due to the crystal defect or the amorphous structure. When the carrier is trapped in the trapping center, the potential of the crystal grain boundary increases and becomes a barrier against the carrier; therefore, the transporting property of the carrier decreases.
In view of the above problem, a technique relating to the crystallization of the semiconductor film with the use of the continuous wave laser has attracted attention recently. In the case of the continuous wave laser, unlike the conventional pulsed laser, when the laser beam is scanned in one direction to irradiate the semiconductor film, it is possible to grow a crystal continuously in the scanning direction and to form an aggregation of crystal grains including a single crystal extending long in the scanning direction.
To increase the throughput of the laser annealing, it is necessary to shape the continuous wave laser beam into linear by an optical system. The important point in shaping the laser beam is the homogeneity of the energy density distribution of the beam spot in a major-axis direction (also referred to as a long-side direction). The energy density distribution in the major-axis direction affects the crystallinity of the semiconductor film crystallized by the laser annealing, and moreover affects the characteristic of a semiconductor element formed using the semiconductor film crystallized thus. For example, when the beam spot has Gaussian energy density distribution in the major-axis direction, the characteristic of the semiconductor element formed using such a beam spot also varies so as to have the Gaussian distribution. Therefore, in order to secure the homogeneity of the characteristic of the semiconductor element, it is desirable to homogenize the energy density distribution of the beam spot in the major-axis direction. The beam spot having homogeneous energy density distribution in the major-axis direction has an advantage of high throughput because the beam spot can be made longer in the major-axis direction.
To homogenize the energy density of the linear beam spot in the major-axis direction, it is necessary to use an optical element such as a cylindrical lens or a diffractive optical element. However, these optical elements for homogenizing the energy density have a problem in that the adjustment is complicate because they require advanced optical design in consideration of a wavefront and a shape of the beam spot.
Moreover, the semiconductor film can be crystallized more effectively when the absorption coefficient of the laser beam to the semiconductor film is higher. In the case of a YAG laser or a YVO4 laser, the second harmonic has higher absorption coefficient than the fundamental wave to a silicon film having a thickness of several tens to several hundred nm, which is usually used in a semiconductor device. Therefore, usually, the harmonic having shorter wavelength than the fundamental wave is used in the laser crystallization for manufacturing a semiconductor device, and the fundamental wave is hardly used. The harmonic can be obtained by converting the fundamental wave by the non-linear optical element.
However, the continuous wave laser has lower output power per unit time than the pulsed laser. Therefore, the density of photon to time is also low, and the conversion efficiency into the harmonic by the non-linear optical element is also low. Specifically, in contrast with the pulsed laser having a conversion efficiency of approximately 10 to 30% the continuous wave laser has the conversion efficiency of approximately 0.2 to 0.3%. The continuous wave laser has another problem in that the resistance of the non-linear optical element is much lower than that in the pulsed laser because the continuous wave laser continuously gives burden to the non-linear optical element.
Therefore, a laser beam having the harmonic emitted from the continuous wave laser per unit time has low power, and it is difficult to increase the throughput by enlarging the area of the beam spot, compared with the pulsed laser beam. For example, a continuous wave YAG laser can provide the fundamental wave with an output power as high as 10 kW but provide the second harmonic with output power as low as 10 W. In this case, the area of the beam spot must be narrowed as small as 10−3 mm2 in order to obtain the energy density required to crystallize the semiconductor film. As thus described, the continuous wave laser is inferior to the pulsed excimer laser in throughput, and this is one factor to decrease the economical efficiency in mass production.